The present invention relates to vacuum gauges and more particularly to ionization gauges for use over a wide pressure range.
Ionization gauges typically comprise a source of electrons (cathode), an accelerating electrode (anode) to provide energetic electrons, a collecting electrode (collector) to collect the ions formed by electrons impacting on gas molecules within the gauge and an envelope or outer electrode surrounding the other electrodes. Ideally the number of positive ions collected within the gauge is directly proportional to the molecular gas density within the gauge. However, in prior art gauges there are numerous factors which cause the number of positive ions collected not to be strictly proportional to the density. Also, the production of undesirable extraneous currents in the gauge, which are independent of gas pressure, tend to present a practical barrier to measurement of very low pressure. Build-up of positive ion space charge at higher pressures leads to loss of ions collected by the ion collector which tends to set an upper limit on the pressure which can be measured.
The primary reason that ion current collected is not proportional to gas density in prior art gauges is that the number of ions produced per electron emitted is not constant at any given pressure. The prior art gauges have not caused emitted electrons to produce a proportional number of ions at any given pressure.
Extraneous currents principally result from a so-called X-ray effect. Bombardment of the anode by electrons produces soft X-rays. Some soft X-rays impinge on the collector, thereby producing a photo-electron current which adds to the ion current in the collector. The photo-electron current and the ion current are not distinguishable from one another in the ion current measuring circuit. Thus, the photo-electron current establishes a lowest practical limit beyond which meaningful ion current measurement cannot be made.
Vacuum gauges are known which have reduced the X-ray effect by several orders of magnitude and with special precautions still lower. One such gauge, commonly referred to as the "Bayard-Alpert (BA) gauge", is disclosed in U.S. Pat. No. 2,605,431. See also U.S. Pat. Nos. 4,636,680 and 4,714,891 assigned to the assignee of the present application. All of the foregoing U.S. Patents are incorporated herein by reference. The BA ionization gauge is widely used. However, because low pressure gauge calibration is a very expensive and time-consuming procedure, most BA gauges are used as manufactured, and are typically not subjected to calibration before use. Thus, it is highly desirable that the gauge sensitivity be reproducible gauge to gauge and stable from measurement to measurement in the same gauge.
Unfortunately, the sensitivity of commercially available BA gauges tends to be neither reproducible, nor stable. It has been found that typical commercially available BA gauges exhibit substantial differences in sensitivity from gauge to gauge. See K. E. McCulloh and C. R. Tilford, J. Vac. Sci. Technol. 18 994 (1981). Further, it has been noted that the sensitivity of typical BA gauges tends to drift by, for example, as much as 1.4% per 100 operating hours when kept at vacuum. Moreover, changes in sensitivity of up to 25% occur when the gauge is briefly exposed to the atmosphere and then operated in vacuum. See K. F. Poulter and C. J. Sutton, Vacuum 31 145 (1981).
For repeatable and stable sensitivity at a given emission current and over a given pressure range, it has been determined that:
1. The fraction of the electron emission current which is effective at producing ions remains constant over time and from gauge to gauge. PA1 2. The instantaneous ionizing energy of electrons of corresponding distances in their trajectories is constant over time and from gauge to gauge. PA1 3. The total electron path in the ion collection volume within the anode is constant over time and from gauge to gauge. PA1 4. The ion collection efficiency is constant over time and from gauge to gauge. PA1 1. Electrons are emitted from the cathode in many different directions. This is the situation in the widely used B-A gauge where computer modeling shows that prior art cathode-anode geometries cause most electrons to acquire substantial tangential components of velocity. Thus, many different shaped electron trajectories exist. PA1 2. Electrons are emitted in all directions and then redirected generally toward the anode as in Redhead's U.S. Pat. No. 3,743,876. This is an improvement on the B-A gauge but still results in a great variety of trajectory shapes. PA1 3. Electrons are emitted from the cathode and then focused through an entrance slot in the anode by suitable focusing electrodes. This is the arrangement used in above-mentioned U.S. Pat. No. 4,636,680 of which applicant is a co-inventor. Computer modeling shows that the electron stream converges on a narrow slot in the anode. Once inside the anode volume, the electron stream diverges producing a great variety of electron trajectories. PA1 4. Electrons are launched from a narrow strip cathode in parallel paths directly at the ion collector located on the axis of symmetry of the anode. Computer modeling shows that this method of launching electrons produces a wide variety of electron trajectories.
These fundamental requirements are not well-satisfied in most prior art BA gauges. Although many of these requirements are considered in above-mentioned U.S. Pat. Nos. 4,636,680 and 4,714,891, the ion gauges of the present invention constitute improvements with respect to the gauges disclosed in these patents.
In a typical BA gauge, the electric field varies from place to place in the gauge. Accordingly, the ionizing energy that an electron acquires depends both upon the particular trajectory of the electron and the instantaneous position of the electron along the trajectory. Electron paths vary greatly depending on where on the cathode and in which direction the electron is emitted. See, for example, L. G. Pittaway, J. Phys. D. Appl. Phys. 3 1113 (1970).
Attempts have been made to control the divergence of the emitted electron stream from the cathode to anode. For example, a special electrode has been placed behind the cathode for this purpose. Such a gauge is described in U.S. Pat. No. 3,743,876 issued to P. A. Redhead, which patent is also incorporated herein by reference.
Computer stimulation of electron trajectories using Redhead's design shows some improvement in focusing more of the electrons into the anode volume but there still exists a huge diversity of electron trajectories mainly because many electrons are launched tangentially.
Ionization gauges have been made which exhibit sensitivities which are reproducible and stable to better than .+-.2% over an 18-month period. However, these transducers are elaborate, complex and costly devices not suited for general use and are incapable of measuring very low pressures. See K. F. Poulter et al, J. Vac. Sci. Technol. 17 679 (1980).
Determining the actual trajectories of individual electrons or ions in any given electrode geometry is a difficult task at best. Thus, resort is typically made to computer simulations of the potential gradients which exist in a given electrode geometry and compute the expected trajectory of a charged particle based on the known physical properties of the charged particle. Such techniques of computer modeling or simulation of charged particle trajectories are well-known in the art. In the present invention, applicant used a sophisticated program to provide the charged particle trajectories described and shown hereinafter. This program was funded by the U.S. Department of Energy. All of the trajectory results shown hereinafter may readily be duplicated by modeling the same electrode geometries and electrode potentials to the same accuracy with this or any comparable program.
Applicant has found using computer modeling that four distinct prior art modes of controlling electron trajectories can be distinguished. Each mode produces a great variety of electron trajectories in B-A geometry: